1. Field of the Invention
The present invention relates to a magneto-optical disk and a method for magnetizing the disk, and more particularly to a magneto-optical disk which is initially magnetized before shipment or prior to being used and to a method for magnetizing the disk.
2. Related Background Art
Thermomagnetic writing (or recording) is mainly employed for recording data in a magneto-optical disk. It is a method for writing data at a Curie point on a recording magnetic layer utilizing a temperature rise under irradiation of a laser beam. The recording is effected by forming regions having different magnetic properties on the disk. This means that a geometric pattern corresponding to information to be recorded is formed on the disk. The magneto-optical disk is characteristic particularly in that the thus recorded pattern can be optically read utilizing the magneto optical effect.
The magneto-optical disk is principally used in the field of electronic information processing, for example in computers at present. Therefore, the recorded pattern is binary code data. Specifically, the pattern is recorded in the form of a string of pits, each of which is a closed region having a shape of a circle, ellipse or elongate ellipse. The current majority of magneto-optical disks utilize light, specifically a laser beam for writing (recording) or reading (reproduction), as described above. Namely, the data writing is effected by the thermomagnetic writing through light absorption, heating and temperature rise in a portion irradiated by a laser beam, and the written data is reproduced or detected utilizing the magneto-optical effect on reflected light or on transmitted light from the irradiated portion. The recording medium is moved relative to the laser beam (i.e., the laser-beam-irradiated portion as described above relatively moves on the recording medium) to perform conversion of time-series electronic data into a geometric (pit) pattern on the recording medium or to perform reverse conversion. Specifically in case of the magneto-optical disk, a disk recording medium (hereinafter referred to simply as a disk) is rotated at a constant linear velocity or at a constant angular velocity, whereby the relative motion is made between the laser beam and the disk. Therefore, a data string (pit string) is formed in the geometric pattern corresponding to the time-series data in the tangential direction of disk on the disk. During the disk rotation, the laser beam is also shifted in the radial direction of disk, whereby the data string of the geometric pattern is also expanded in the radial direction of the disk. This expansion may be done by conventional procedure, for example in a spiral pattern as in (acoustic) record disks of well known arrangement, or in a pattern of concentric circles as known in the field of floppy magnetic recording disks. In other words, the two-dimensional geometric pattern on the disk is signified as a one-dimensional geometric pattern string which can be converted into time-series data. This string is called hereinafter an information track. There are various disks known as described, including a disk with a single spiral information track extending thereon and a disk with concentric circles as information tracks. Normally, the information track often has a three-dimensional geometric structure on the disk. Namely, there are lands and grooves formed on the surface of disk. The magneto-optical pits are formed on the lands. The lands extend in a spiral pattern or in a concentric pattern on the disk, forming the information track(s). On the other hand, the grooves are formed as projected or recessed on either side of land. The grooves constitute a sort of diffraction grating together with the lands because of their height difference, so that diffraction-reflected light emerges therefrom upon irradiation on the disk with the laser beam, as detailed hereinafter. While a drive device receives this diffraction-reflected light to detect a relative position of the laser-irradiated portion relative to a land or a groove on the disk, it carries out scanning of a laser beam along the land portion. This tracing scan is often called a land trace and the tracing control is referred to as tracking servo.
It is evident that data density increases on the disk as the area of a laser-irradiated portion for writing or reading on the disk decreases and that with the decrease in area of the irradiated portion, an amount of data which can be handled within a unit time at a constant rotation speed of disk also increases. The laser-irradiated portion is hereinafter called and a spot. For the above reason, it is preferred for magneto-optical disks that the spot is formed as small as possible. In the currently available systems, the laser beam is focused in a spot of diameter of about 1 to 2 .mu.m to irradiate the disk thereby.
The thermomagnetic recording on the magneto-optical disk is further described below. The magneto-optical information recording is identical to the other methods of magnetic information recording in that the recording is made by fixing the orientation of spontaneous magnetization in a magnetic recording medium which is ferromagnetic. In detail, the aforementioned geometric pits are discriminated one from another by the orientation of spontaneous magnetization. Relatively scanning the spot on the medium disk as described above, a part of a beam in the spot is absorbed by the spot area on the disk at a certain time to heat the portion and raise the temperature thereof. As well known in the fields of the magnetism physics and the magnetic recording technology, a ferromagnetic material changes the spontaneous magnetization (moment) with a change in temperature and particularly the spontaneous magnetization disappears over the critical temperature Tc (this critical point of phase change is called a Curie point in the field of magnetism). Thus, when the temperature rise brings the portion over or near the Curie point, the spontaneous magnetization will disappear or become very weak. Then, applying a magnetic field with suitable strength from the outside, the spontaneous magnetization of the portion can be aligned with a certain direction.
Describing in more detail, the temperature-increased portion, as the spot moves is cooled to finally reach the ambient temperature around the disk. If the portion is kept within a region of influence of the external magnetic field thus applied before the magnetic material recovers a certain strength of spontaneous magnetization in the cooling process, the spontaneous magnetization is fixed in a certain direction depending upon the direction of an applied magnetic field to form a fixed area. The magnetization is fixed in the direction perpendicular to the surface of the magnetic layer in the field of the art.
As described above, the thermomagnetic recording utilizes the temperature dependency of spontaneous magnetization as follows: a temperature rise makes a portion have a lower strength of spontaneous magnetization than that at room temperature, the magnetization in the portion is arranged in a certain direction in the applied magnetic field (which has a lower intensity than that at room temperature) and the portion is cooled to fix the spontaneous magnetization therein. Therefore, the geometric shape of a thus produced record-magnetized area (pit) is influenced by a temperature distribution on the disk made by the spot or the applied magnetic field.
Meanwhile, the reading is highly dependent upon the geometric shape of pit, as well known. The reading from a magneto-optical disk uses reflected light (or transmitted light) from the spot of a laser beam for reading, as described above. (Although the following description concerns only the reflected light, the description can be applicable to the transmitted light.) An additional feature of the magnetic recording member in the magneto-optical information recording medium is that the member is made of a material having a great magneto optical effect (which is well known as the Kerr effect for reflected light and the Faraday effect for transmitted light).
As generally well known in the fields of the art and the optical physics, such a material has two types of index of refraction with proper circularly-polarized components, so that the reflected light or transmitted light may have polarization components different from those of incident light. This change also depends upon the direction of incident light (polarization) and the direction of spontaneous magnetization. The polar Kerr effect is presently used by the majority of magneto-optical disks. In such magneto-optical disks, the direction of spontaneous magnetization is the normal direction or the reverse direction parallel to the traveling direction of an incident (reflected) beam, the reflected light is elliptically polarized with the major axis inclining to the incident polarization of the incident light linearly polarized, and the major axis of elliptic polarization is inclined in two different directions depending upon the normal or reverse direction of spontaneous magnetization. Accordingly, the direction of magnetization can be read out from the spot portion on the medium by detecting the difference in inclination direction by a polarization detecting device. The polarization detecting device can apply any one of many well known detecting methods to the detection.
Generally, a spot includes a pit region in which the magnetization is arranged by writing in a certain direction, and the other region. Among the reflected light from the spot portion, a reflected beam from the pit region gives an output through the above-described polarization detection and a reflected beam from the other region than the pit region another output. The total output is a sum of these outputs over the entire area in the spot portion. In particular, it is apparent that the minimum and the maximum output signals are given when a pit center is located at the center of a spot portion and when the spot portion includes no pit, respectively.
With a relative scan using a spot as described above, the area of a pit portion changes in the spot. Therefore the detected output signal also changes with the area change. Using this fact, the geometric information pattern on the medium is converted into a time-series (electric) information string.
The recording technique for a magneto-optical disk arranges the magnetization in a specific direction in a local area on the recording medium, thus performing the thermomagnetic recording. There are roughly two methods known for the localization. One of them is a method for controlling the temperature rise by modulating the intensity of recording laser beam while the other is a method for controlling the localization by modulating the applied magnetic field (direction of field). These two methods have respective advantages and disadvantages and are different from each other for example in respect of a shape of a of formed pit. In case the modulation of a laser beam is carried out using binary signals obtained by encoding ON and OFF, a teardrop-shaped pit is formed. This is understood as follows. A high temperature portion appears locally on the medium with an ON signal of the laser. The temperature distribution in this portion is represented by four-cornered isotherms, which are prolate in the moving direction of the spot upon scanning and are wide at the tail due to heat accumulation while being tapered off at head like a teardrop. As described above, the spontaneous magnetization disappears or is weakened near or over the Curie point (or compensation point) and the portion is cooled under application of an external magnetic field with an OFF signal of the laser or after passing the spot to fix the magnetization near a certain temperature. This is the reason why the pit has the shape of a teardrop projecting at the both ends of head and tail reflecting the isotherms upon temperature rise. In contrast, in case the direction of magnetic field is modulated according to binary information, an arrow-tail pit is formed with the head projecting in the opposite direction to the spot movement, reflecting isotherms upon cooling after passing the spot. This difference is caused by a difference in cooling process. In more detail, the laser modulation method has a cooling process in which the head of pit is formed after passing the spot while the tail of pit before passing the spot (after the laser is turned off). On the other hand, the magnetic modulation method has a cooling process in which the both ends of pit are formed after passing the spot.
This difference of pit shape causes the following difference in reading signal. If writing or reading is carried out without any correction for ON, OFF repeating signals with the same length, the laser beam modulation method is likely to have an offset difference due to a frequency difference. This is because a ratio of shape or area between a pit portion and a non-pit portion within the spot changes depending upon the frequency. This results in decreasing the margin when the information obtained in reproduction is decoded into binary signals. Therefore, correction might be required according to the frequency in writing in some cases. This tends to be affected by variations in writing laser control, thermal sensitivity of medium, etc.
In contrast, the magnetic modulation method is unlikely to cause the offset due to the frequency difference and thus requires no specific correction in writing, because the both ends of a pit are formed in the same thermal process. Let us consider these phenomena in respect of heat. In the magnetic modulation method, the spot as a heat source moves at an almost constant speed in the time scale for pit formation to produce a state in thermally approximate equilibrium or a state in non-equilibrium but in a very stable state. In the optical modulation method, the time scale of 0N and OFF of laser is equivalent to the time scale of pit formation (even with the writing correction), and therefore it is highly dependent upon heating area or conduction. Speaking from the viewpoint of pit formation directly related to the reading signal, it is very difficult to stably produce a constant shape for the head or the tail of pit or for several adjacent pits. This means that an important point is to control the laser as a heat source within a very short period corresponding to the pit shape.
Also, there is another difference between the two methods. In the optical modulation method, the size of a formed pit is substantially determined by the size of a laser beam spot utilized as a heat source, while in the magnetic modulation method, the pit length in the spot scanning direction or in the track direction is determined by the time interval of an applied external field and the scanning speed of the spot. Thus, the magnetic modulation is effective to form a pit with shorter pit length as compared with the spot size. Since the magnetic modulation is effected while inverting the applied field at high speed as described, it is difficult for the field generating means (which is theoretically an electromagnet referred to as a magnetic head) to produce a strong magnetic field. Therefore, the field generating means is usually mounted on an aerodynamic slider so that the magnetic field can be applied only to the area near the laser spot while a constant distance is maintained with respect to the recording surface of a disk magnetic layer.
The conventional magneto-optical disks, however, had the following drawbacks.
FIG. 1 is a drawing to schematically show a state of pit formation in information recording on a conventional magneto-optical disk. Numeral 31 designates a recording layer in the magneto-optical disk and 32 a writing laser focusing beam. The temperature rises in a portion irradiated by the laser in the recording layer to decrease the spontaneous magnetization. The external magnetic field Hex is applied to the irradiated portion (upward in FIG. 1) to align the magnetization with a determined direction (upward in FIG. 1). As the laser beam moves (or is turned off), the magnetization in the portion is fixed upon cooling to form a magnetized pit. The size of pit produced in this occasion depends upon the strength of external applied field Hex as well as the spot size and the light intensity of the irradiating laser beam on the recording layer. Thus, the strength of magnetic field Hex affects the quality of recorded information signal. The recording layer, however, is also affected in actual pit formation by a leaking field Hp caused by magnetization Mp in the surrounding recording layer in addition to the external magnetic field Hex. Specifically, the conventional magneto-optical disks have grooves for servo on the both sides of each land on which the recording pits are actually formed. The magnetization in the adjacent grooves produces a leaking field, which affects the strength of magnetic field actually applied in pit formation, thereby influencing the recording properties. (Since the grooves are in a region out of a user's recording area, the leaking field cannot be controlled by the drive device.)
Incidentally, the conventional magneto-optical disks are shipped after the entire surface of each disk is initially magnetized in a certain direction. Therefore, grooves adjacent to a land subjected to pit formation are uniformly magnetized and the leaking field from the grooves had the greatest adverse influence on the recording properties (magnetic sensitivity of disk).
Summarizing the above, the drawbacks of conventional magneto-optical disks are listed as follows.
(1) The recording properties, especially the noise properties, change depending upon the leaking field influencing the external magnetic field set by the drive device.
(2) Further, the degree of correction for the leaking field cannot be uniquely determined on the drive side, because the strength of leaking field depends upon the disk. Therefore, matching is difficult for a combination between drive and disk.
(3) The direction of initial magnetization upon shipment could vary in a disk, which may cause a variation of leaking field depending upon a location, resulting in causing a change in recording properties.
The third drawback can be avoided by uniform magnetization upon shipment, but the other drawbacks cannot be avoided.
Further, these drawbacks become outstanding as described below in case of the magneto-optical disk employing the magnetic modulation.
(4) In the magnetic modulation method, a pit is formed by modulating the external magnetic field. Namely, the polarity of external field is inverted according to the information so as to change the polarity of induced magnetization on the recording layer. The difference in polarity produces a pit. In the pit formation, a leaking field adds a DC component to the external field on the recording layer, causing a great change in recording properties, especially in sensitivity of recording field. In the magnetic modulation method in which the external field has to be inverted at high speed, the difference in sensitivity of recording field raises a big problem in improving the density, the transfer rate, etc. of information signals recorded in the disk.